The present invention relates to optical systems, and, more particularly, to Schlieren imaging systems.
Light valves, or spatial light modulators, have been used in conjunction with Schlieren imaging systems for many years in projector applications where large, bright displays of video information are required. In these projectors, the electronic video information is converted into corresponding phase perturbations across a beam of light by the spatial light modulator. The Schlieren imaging system then converts the phase modulations across the light beam leaving the modulator into light intensity variations at a viewing screen by blocking the unmodulated light and passing a large fraction of the light incident to modulated areas on the light valve. An optical printer based on this approach has also been recently proposed and converts the modulated light to printed form by xerography. Light valve projectors have at least two features that are important in display or printing applications. The light valves themselves are electronically addressable in an areal (display) or linear (printing) manner. This feature makes it possible to present electronically generated data in "real time". Secondly, the light valve is used to gate or control the light from a separate, external source. The properties of the light source can thus be chosen independently to meet system size, power, and cost requirements while achieving the desired display or photoreceptor irradiance level.
Since the attainment of the highest possible light level at the final image plane has been a goal of all light valve projector systems, bright, compact light sources such as arc lamps or lasers have traditionally been used together with efficient optical configurations having the highest possible optical throughput. The light modulating characteristics of the light valve and the configuration of the Schlieren stop that is used with it have a critical impact on the attainable optical efficiency. The stop must be tailored to the light valve in order to both efficiently block the background unmodulated light and pass a large fraction of the signal energy that is diffracted from the modulated areas of the light valve.
Several different light valve technologies have been utilized to date, each one incorporating a different type of stop plane discrimination. A brief overview of some of these technologies will be presented together with proposed improvements to the optical system that has been used with previous cantilever beam light valves.
The oldest of the light valve technologies is the electrostatically deformable oil film. It has been incorporated into both the Eidophor theatre projector system and the General Electric color television projector ("Color Television Light Valve Projection Systems," IEEE International Convention, Session 26/1, 1-8 (1973)). In both systems, a continuous oil film is scanned in raster fashion with an electron beam that is modulated so as to create a spatially periodic distribution of deposited charge within each resolvable pixel area on the oil film. This charge distribution results in the creation of a phase grating within each pixel by virtue of the electrostatic attraction between the oil film surface and the supporting substrate, which is maintained at constant potential. This attractive force causes the surface of the film to deform by an amount proportional to the quantity of deposited charge. The modulated light valve is illuminated with spatially coherent light from a Xenon arc lamp. Light incident to modulated pixels on the oil film is diffracted by the local phase gratings into a discrete set of regularly spaced orders which are made to fall on a Schlieren stop consisting of a periodic array of alternating clear and opaque bars by part of the optical system. The spacing of the Schlieren stop bars is chosen to match the spacing of the diffracted signal orders at the stop plane so that high optical throughput efficiency is achieved. Light that is incident to unmodulated regions of the light valve is blocked from reaching the projection lens by the opaque bars of the Schlieren stop. Images formed of unmodulated areas on the light valve by the Schlieren imaging system on the projecting screen are therefore dark, while the phase perturbations introduced by the modulated electron beam are converted into bright spots of light at the screen by the Schlieren projector.
The Eidophor and the General Electric light valve projector are commerically available products that have been used for educational, entertainment, military, and NASA applications where a display suitable for a very large audience is required. They are both large, heavy, and expensive and thus unsuitable for high volume, low cost display or printer applications.
Several efforts have been made to improve on the size, cost, and manufacturability of the oil film projectors ("Survey of Developmental Light Valve Systems," IEEE International Convention, Session 26/2, 1-10 (1973)). Many of these efforts have concentrated on replacing the oil film by a thin, reflective membrane that is mounted to the faceplate of a CRT by means of a support grid structure. These light valves are thus also addressed by a raster scanned electron beam. An electrostatic force of attraction is generated between the charge deposited on the glass faceplate by the electron beam and the membrane, which is held at constant voltage. This attractive force causes the membrane to sag into the well formed by the grid structure, thereby forming a miniature spherical mirror at each modulated pixel location. The light diffracted from this type of modulated pixel is concentrated into a relatively narrow cone that is rotationally symmetric about the specularly reflected beam. This type of light valve must thus be used with a Schlieren stop that consists of a single central obscuration positioned and sized so as to block the image of the arc source that is formed by the optical system after specular reflection from unmodulated areas of the light valve. Modulated pixels give rise to a circular patch of light at the Schlieren stop plane that is larger than the central obscuration, but centered on it. The stop efficiency, or fraction of the modulated pixel energy that clears the Schlieren stop, is generally somewhat lower for projectors based on deformable membranes than it is for the oil film projectors discussed above.
Light valve projectors based on deformable membranes have never been turned into commercial products for at least two reasons. The fabrication process is very susceptible to defects that result when even small, micron sized particles are trapped between the membrane and the underlying support grid stucture. The membrane would form a "tent" over these trapped particles whose lateral extent is much larger than the size of the particle itself, and these tents would in turn be imaged as bright defect spots by a Schlieren imaging system. There are also addressing problems caused by slight misalignments between the electron beam raster and the pixel support grid structure. Such misalignments would cause image blurring and nonuniformity in display brightness.
Another light valve projection system incorporating a central obstruction type of Schlieren stop has recently been devised specifically for an optical printing application. ("Linear Total Internal Reflection Spatial Light Molecular for Laser Printing," Proc. SPIE, 299, 68-75 (1981)). The light valve for this application is a hybrid one consisting of a silicon drive chip having a linear array of address electrodes pressed into intimate contact with a polished face on an electro-optic crystal. Laser light that is collimated in the direction perpendicular to the electrodes is incident to the contact surface at an angle greater than the critical angle for the crystal material and is thus totally internally reflected from the interface. Fringing fields are created in the electro-optic material by applying voltage differences between adjacent pairs of electrodes. These fields change the refractive index of the crystal in the vicinity of the electrodes, and the resulting index gradient diffracts light out of the specular beam in one dimension. A field lens is used to focus all the unmodulated, or specularly reflected light onto a central obscuration at the Schlieren stop plane. Light diffracted from the modulated pixels partially clears this obscuration and is brought to focus at the photoreceptor by the imaging lens. The diffraction efficiencies quoted for this system were similar to those of the deformable membrane light valves. The photoreceptor drum rotates beneath the image of the linear array of light valve pixels to generate a two dimensional page of text.
The silicon integrated circuit addressing scheme used for this light valve is more practical for high volume, low cost printer applications than the previous electron beam addressed light valves. However, the light valve is highly susceptible to fabrication problems due to its hybrid nature. The fringing field strength, and hence the amount of light diffracted from modulated pixels, is sensitive to changes in the air gap thickness between the address electrodes and electro-optic crystal surface of less than 0.1 micron. Hence, even very small particles trapped between the crystal and electrode structure could cause illumination nonuniformity problems at the photoreceptor. The system optical response for pixels located at the boundary between modulated and unmodulated areas of the light valve is also significantly lower than the response for pixels near the middle of a modulated region due to the nature of the addressing technique. A commercially available printer based on this technology has not been introduced to date.
The remaining light valve technology that has received significant interest for projection applications is the cantilever beam spatial light modulator. This type of light valve consists of an array of micromechanical cantilever beams that can be electrostatically deflected. The first work on this technology was done by Westinghouse ("The Mirror-Matrix Tube: A Novel Light Valve for Projection Displays," IEEE Trans. on Electron Devices, ED-22, 765-775 (1975), U.S. Pat. No. 3,886,310 (May 27, 1975)) using a cloverleaf beam geometry. Each cloverleaf consists of four cantilever beams which are joined at one corner to a common central support post. An area array of pixels is fabricated on the sapphire faceplate of a CRT and is thus addressed by a scanning electron beam. The electron beam deposits charge on the cantilever beams themselves, thereby creating an electrostatic force of attraction between the cloverleaves and an underlying electrode grid structure that is held at constant potential. This attractive force causes the cantilever beams to bend at their hinge points toward the electrodes.
Deflected beams are imaged onto a projection screen as bright points of light in the following manner. Light that is reflected from flat, unmodulated areas of the device appears at the Schlieren stop plane as a cross-shaped diffracted background. The two arms of the cross are generated by diffraction from the lithographically defined edges of the pixels and are thus oriented prependicular to those edges. An opaque, cross-shaped obscuration is therefore placed at the center of the Schlieren imaging lens pupil to prevent this diffracted background from reaching the projection screen. Light reflected from deflected pixels is deviated out of the unmodulated beam by an angle equal to twice the pixel deflection angle. Since the pixels are hinged at a corner, they bend at a 45 degree angle with respect to their edges, and the signal energy appears at the Schlieren stop plane as patches of light in the unobscured quadrants of the imager pupil. The use of this so-called "45 degree stop discrimination" thus permits the signal energy to be placed in the region of the lens pupil where the diffracted background is minimized. It is a key feature of the Westinghouse approach that results in a good contrast ratio at the Schlieren image of a cantilever beam light valve even with small pixel bend angles on the order of a few degrees. The lateral separation that is obtained between the diffracted background and deflected signal energy at the Schlieren stop plane for projectors based on cantilever beam light valves also guarantees good optical throughput or high stop efficiency provided the achievable pixel bend angle is larger than the angular size of the light source as seen by the light valve.
The Westinghouse Mirror Matrix Tube offers no significant advantage over conventional projection CRT's in terms of either cost or performance. It also suffers from the problem of maintaining precise registration between the electron beam raster and the array of spatial light modulator pixels. IBM has sought to bypass these problems, which are inherent to any electron beam addressed light valve, by developing a cantilever beam device that could be integrated with its own drive circuitry onto a silicon chip. Two different implementations have been described in the literature. The first ("Dynamic Micromechanics on Silicon: Techniques and Devices," IEEE Trans. on Electron Devices, ED-25, 1241-1250 (1978)) involves the use of a 16 element linear array of "diving board" type pixels. This device does not have on-chip addressing, and it does not incorporate 45 degree stop discrimination into its pixel design. The device is laser illuminated and imaged onto a ground glass screen by means of a Schlieren imaging system that uses a scanning galvanometer mirror to generate an alphanumeric message on the screen. Few details were given regarding the optical performance of the system except that an 8:1 contrast ratio was achieved. The second implementation was described in U.S. Pat. No. 4,229,732 (Oct. 21, 1980). This device consists of an area array of cantilever beam pixels, each of which is comprised of a single flap hinged at one corner. Light reflected from deflected pixels on this device would therefore be directed into only one quadrant of the Schlieren stop plane in the Westinghouse projector. The patent suggests that the light valve be used in display applications with "standard Schlieren image projection systems" appropriate for this light valve technology.
The cantilever beam light valve technology seems to be the most appropriate one for low cost, high volume Schlieren projection applications if it can be successfully fabricated on a silicon chip with an architecture having good optical properties. Such a device could be fabricated with MOS technology to include on-chip address electronics, and it would have a relatively low susceptibility to particulate contamination. Westinghouse has also shown that good contrast ratio and a high stop efficiency can be obtained for reasonable pixel deflection angles if 45 degree stop discrimination is utilized. The only projection system and Schlieren stop configuration that has been proposed for use with cantilever beam light valves incorporating 45 degree stop discrimination is the one used by Westinghouse. This system is believed to have the following fundamental limitations in terms of attainable optical performance.
(1) The aperture diameter of the imaging lens must be larger than is necessary to pass the signal energy alone. Hence the speed of the lens must be relatively high (or, equivalently, its F-number must be relatively low) to pass all the signal energy around the central Schlieren stop obscuration.
In addition, the signal passes through the outer portion of the lens pupil in this imaging configuration. Rays of light emanating from any given point on the light valve and passing through the outermost areas of an imager lens pupil are the most difficult ones to bring to a well-corrected focus during the optical design of any imaging lens. When the outer rays are brought under good control, the rays passing through the center of the imager lens are automatically well corrected. Hence a greater level of optical design complexity is required of the imaging lens by the Westinghouse configuration.
(2) The field angle over which the imaging lens can form well corrected images of off-axis pixels on a cantilever beam light valve is also restricted by the use of the Westinghouse system. Any lens design task involves a compromise between the speed of the lens and the field angle it can cover with good image quality. Fast (low F-number) lenses tend to work over small fields, while wide angle lenses tend to be relatively slow (high F-number). Since the Schlieren imager must be well corrected over its entire aperture, and since this aperture is larger in diameter than is required to pass the image forming light, the field angle that can be covered by the lens is smaller than it could be if a different imaging configuration could be devised in which the signal was passed through the center of an unobscured, smaller diameter lens.
(3) For an imager lens having a given finite speed, the use of the Westinghouse Schlieren stop configuration also limits the size of the light source that can be utilized. This in turn limits the irradiance level that can be delivered to a projection screen or a photoreceptor at the image of a deflected pixel. This irradiance level, or the delivered power per unit area, depends on the product of the radiance of the light source, the transmittance of the optical system, and the solid angle of the cone of image forming rays of light. The source radiance is determined only the the particular lamp that is used. The optics transmittance depends on the stop efficiency for the particular light valve/Schlieren stop configuration and surface transmission losses. But the solid angle of the image forming cone of light is directly proportional to the area of the imager lens pupil that is filled with signal energy. The use of a Schlieren stop that obscures the central area of the imager lens pupil limits the useable pupil area and thus the image plane irradiance level that can be obtained for a lens of a given speed and a source of a given radiance.